It probably was opened at one time but was put into the book and being squeezed between the pages for so long it stuck back together. So technically you're just opening a stuck envelope so go ahead as it has already been opened. Pam

Glad you opened it! You would've looked in the envelope had it already been slit, and it's not like you took it out of their mailbox.I'm reading your book and am delighted with all the buried treasures.

Wish I was a dealer, but am only a small-time, low-cost collector. However, in two of the copies from a complete set of the Jalna novels, I found letters from the author to the books' former owner. That's been my best find.

And looking through one of my old books the other day, I found a re-imbursement check for about $6. It expired in the 80s.